a. Field of the Invention
This invention generally relates to pipe couplings of the type which are particularly adapted for underwater connection of pipelines and the like.
The following U.S. patents are generally illustrative of the state-of-the-art of the pipe couplings of the type over which the present invention may be considered an improvement: U.S. Pat. Nos. 3,986,728; 3,977,702; 3,830,526; 3,784,234; 3,713,675; and 3,713,204; for example. All of the aforesaid types of couplings generally utilize elastomeric seals, such as rubber, which are arranged for application of a compressive force thereto for the purpose of deforming the seal to sealing engagement between the pipe and the coupling member.
B. Description of the Prior Art
As the offshore exploration for oil has continued and as oil and hydrocarbon fluids are produced from deeper formations in such offshore locations, the pipe and associated coupling members are thereby exposed to higher temperatures of the fluid being produced and flowed through the pipeline, and to increased variations between the temperature of the fluid passing through the pipeline and the ambient temperature exterior of the pipeline. For example, it is not uncommon for hydrocarbon fluids to flow through such lines at temperatures in excess of 150.degree. F., and on occasion as high as 240.degree. F. In addition, it is not unusual for the flow of such hot materials through such lines to be of a cyclical nature wherein periods of flow are interrupted by periods of non-flow, such that the pipeline and associated coupling members are repeatedly heated and cooled between the elevated temperatures of the flowing fluid and the ambient temperature of the water exterior of the pipeline. It has been found that these extremely high temperatures and the cyclical nature of the temperature to which such pipelines and couplings are exposed present serious problems with maintaining the aforesaid elastomeric seals properly sealed at all times. The result has been that such frequent temperature changes and high temperatures have caused certain prior art couplings utilizing elastomeric seals to fail or perform in a substandard fashion after a relatively short period of time. If the elastomeric seal is compressed and "set" at a low temperature and the temperature is then increased to some higher temperature the differential thermal expansion between the elastomer of the seal and the steel body will cause the pressure in the elastomer to rise. The magnitude of the pressure rise in the elastomer may be so great that yielding of the pipe will occur or the elastomer will extrude out of the gap beteen the connector body and pipe, or both. The magnitude of the pressure rise in the elastomer packer will be directly proportional to the temperature rise and the difference in the thermal expansion of the elastomer and steel body. The pressure rise in the elastomer will be modulated by the elastic compliance of the pipe, the coupling body and the setting mechanism as well as structural changes in the elastomer (i.e., compression set). Where the pipe is thin or of low strength and the temperature change of sufficient magnitude the pipe can be plastically deformed by the external rubber pressure. If such permanent deformation occurs, a subsequent return of the temperature to the initial condition will cause a permanent loss of the original setting pressure.
Elastomeric materials such as Nitrile rubber expand or increase in volume at a near linear relationship with increases in temperature, particularly above about 150.degree. F. and up to about at least 250.degree. F. Once rubber reaches this temperature range, it does not contract to the same temperature pressure relationship of its original condition if the internal pressure of the rubber goes too high. With excessive temperatures, such as are now encountered in some sub-sea pipeline connector seals, there is a permanent loss in elastic property in the seal material. It has been determined that when the elastomeric packings of the prior art type are subjected to relatively high elevations in temperature in the range heretofore mentioned, excessive pressures develop within the seal and there is an increase in the volume of the packing and a decrease in the viscosity or an increase in the tendency of the rubber to flow. Excessive pressure causes such rubber packings to extrude into the annular space between the pipe and the coupling and out of the confined space in which they are initially held in the sealed position. In addition, the increased volume caused by thermal expansion, unless relieved, may also cause damage to the pipeline at the point of the seal. Upon subsequent cooling of the rubber, the pressure exerted in the confined area is somewhat less than the original and necessary setting pressure because of such loss of rubber and loss of elastic property in the rubber thereby causing the seal to leak.